Optical fiber communication that has been mainly for business use is also becoming widespread for home use. With this movement, there is a demand for a high-performance optical communication device. As optical communication devices for various optical communication systems including optical fibers for home use and local area networks (LAN), silicon-based optical communication devices capable of functioning at optical signal wavelengths of 1330 and 1500 nm exist. The silicon-based optical communication devices are highly promising and, more specifically, can have an optical function element and an electronic circuit integrated on a silicon platform by using a CMOS (Complementary Metal Oxide Semiconductor) technique.
As silicon-based optical communication devices, passive devices such as waveguides, optical couplers and wavelength filters are being widely studied. Also, active devices such as silicon-based optical modulators and optical switches are important elements in optical signal operating means for the above-mentioned optical communication systems, and are attracting a great deal of attention. Optical modulators and optical switches capable of changing the refractive index by a thermo-optic effect of silicon operate only at low optical modulation rates and, therefore, can be used only in apparatuses in which the optical modulation frequency is 1 Mb/sec or lower. An optical modulator using an electro-optic effect is required for apparatuses in which the optical modulation frequency is higher than this value.
No change due to the Pockels effect is observed in the refractive index of pure silicon, and a change in refractive index of pure silicon due to the Franz-Keldysh effect or the Kerr effect is extremely small. Therefore, many of the optical modulators presently proposed and using electro-optic effects use a carrier plasma effect. That is, to change the phase or the intensity of light, the real and imaginary parts of the refractive index are changed by changing the free carrier density in a silicon layer. In an optical modulator using free carrier absorption, the output is directly modulated by changes in absorption of light propagating through silicon. In structures using changes in refractive index, a Mach-Zehnder interferometer is ordinarily used. A light intensity modulation signal can be obtained by causing interference between phase differences in two arms.
The free carrier density in optical modulators can be changed by injection, accumulation, removal or inversion of free carriers. Many of such optical modulators studied heretofore have low optical modulation efficiency and a length of 1 mm or more necessary for optical phase modulation and need an injection current density higher than 1 kA/cm3. Realizing a reduction in size, a high degree of integration, and a reduction in power consumption of an optical modulator requires a device structure capable of obtaining high optical modulation efficiency. The realization of a device structure capable of obtaining high optical modulation efficiency enables reducing the length necessary for optical phase modulation. In a case where the size of an optical communication device is large, the device is vulnerable to the influence of temperature on the silicon substrate and the electro-optic effect to be obtained may be cancelled out by a change in refractive index of the silicon layer due to a thermo-optic effect.
FIG. 1 shows an example of related art for a silicon-based optical modulator using a rib waveguide formed on a SOI (Silicon on Insulator) substrate. Buried oxide layer 32 and intrinsic semiconductor 31 including a rib-shaped portion are successively laid on substrate 33. On opposite sides of the rib-shaped portion of intrinsic semiconductor 31, p+ doped semiconductor 34 and n+ doped semiconductor 35 are respectively formed at certain distances therefrom. Each of p+ doped semiconductor 34 and n+ doped semiconductor 35 is formed by performing high-concentration doping of portions of intrinsic semiconductor 31. The structure of the optical modulator shown in FIG. 1 is a PIN (P-intrinsic-N) diode. When forward and reverse bias voltages are applied to the PIN diode, the free carrier density in intrinsic semiconductor 31 is changed and the refractive index is changed by a carrier plasma effect. In this example, electrode contact layer 36 is disposed on one side of the rib-shaped portion of intrinsic semiconductor 31, and p+ doped semiconductor 34 mentioned above is formed at a position opposite from electrode contact layer 36. Similarly, another electrode contact layer 36 is formed on the other side of the rib-shaped portion of intrinsic semiconductor 31, and n+ doped semiconductor 35 is formed at a position opposite from electrode contact layer 36. The waveguide including the rib-shaped portion is covered with oxide cladding 37. In the above-described PIN diode structure, high-concentration doping can be performed so that the carrier densities in semiconductors 34 and 35 are about 1020/cm3.
When an optical modulation operation is performed, a forward bias voltage is applied to the PIN diode from a power supply connected to electrode contact layer 36 to inject free carriers into the waveguide. At this time, with the increase of free carriers, the refractive index of intrinsic semiconductor 31 is changed, thereby performing phase modulation of light propagated through the waveguide. However, the speed of this optical modulation operation is limited by the lifetime of free carriers in the rib-shaped portion of intrinsic semiconductor 31 and by carrier diffusion when the forward bias voltage is removed. The optical modulator that has the PIN diode structure according to this related art ordinarily has an operating speed in the range from 10 Mb/sec to 50 Mb/sec when a forward bias voltage is applied. An impurity may be introduced into intrinsic semiconductor 31 in order to reduce the carrier lifetime. This enables increasing the switching speed. However, there is a problem in which the introduced impurity will cause a reduction in optical modulation efficiency. Also, the dominant factor that influences the operating speed is RC time constant. With reduction of the carrier depletion layer in the PN (Positive-Negative) junction, the electrostatic capacity at the time of application of a forward bias voltage becomes considerably large. Theoretically, a high-speed operation of the PN junction can be achieved by applying a reverse bias voltage. However, a comparatively high drive voltage or a large element size is required.
Another example of related art is a silicon-based optical modulator disclosed in patent literature 1. This optical modulator has a capacitor structure in which buried oxide layer 32 and the main body region of a first conduction type are successively laid on substrate 33; the gate region of a second conduction type is laid so as to partly overlap the main body region; and thin dielectric layer 41 is formed at the layer interface between the main body region and the gate region. In the following description, “thin” denotes submicron order (smaller than 1 μm).
FIG. 2 shows a silicon-based optical modulator formed of a SIS (silicon-insulator-silicon) structure according to the related art. The optical modulator is formed on a SOI substrate constituted by substrate 33, buried oxide layer 32 and a main body region. The main body region is constituted by p doped semiconductor 38 formed by doping the silicon layer in the SOI substrate, p+ doped semiconductor 34 formed by high-concentration doping, and electrode contact layer 36. A gate region is constituted by n doped semiconductor 39 formed by doping a thin silicon layer laid on the SOI substrate, n+ doped semiconductor 35 formed by high-concentration doping, and electrode contact layer 36. Oxide cladding 37 is provided in the gap between buried oxide layer 32 and the main and gate regions, and above the main and gate regions.
Change of the carrier density in the doped regions is controlled by means of an external signal voltage. Also, when a voltage is applied to electrode contact layers 36, free carriers are accumulated, removed or inverted on the opposite sides of dielectric layer 41. Optical phase modulation is performed thereby. Therefore it is desirable that the optical signal electric field region and the region in which the carrier density is dynamically controlled from the outside coincide with each other.